The Versatility of Oil Paint
Oil paint was not suddenly discovered but instead evolved through the 15th century. Previously, painters had known that it was possible to obtain a greater softness of transition in handling from the egg-bound tempera paints they used by adding small amounts of walnut oil. As the century progressed, first Flemish artists, such as Jan Van Eyck, and then Italian, such as Antonello da Messina, made this addition routine to make their paints capable of capturing the richness and variety of a developing artistic sensibility. Oilier paints permitted more excellent tonal range and intensity of colour. By the following century, linseed oil was also being used, and the original egg binder was frequently dispensed with.
The pre-eminence of oil paint amongst artist paints since then is explained mainly by its susceptibility to so many different modes of application and combination, from the glazes of the 15th century to the striated impasto of late Titian to the thin stain-like veils of Goya. Its adoption is also due to its robustness as a material, allowing for a generous notion of good practice regarding long-lasting results. If certain basic norms are respected, oil paint will permit varieties of use without rapid failure. These norms are themselves quite ‘fuzzy-edged’; for each, there can be found cases in the past where artists have slightly stretched them, ‘lived dangerously’, and still produced work that has lasted in good shape. But if they are ignored altogether, the dangers are a range of physical or chemical changes entirely unanticipated and unwanted by the artist, which will leave the work, however brilliantly executed, in a state which diminishes or even conceals its original qualities.
This website section offers a summary of such essential advice with additional relevant information. If you want to consider some more profound aspects of the use of paint, go on to the section on Colour and Technique.
Whilst every care has been taken to check the soundness and accuracy of this advice, Michael Harding and his associates cannot be held responsible or liable for problems resulting from its application in circumstances of which they have no knowledge and over which they have no control.
1. Choose the Support Carefully
A support is whatever material onto which the paint of an artwork, including the primer, is applied. Slate, marble, copper, aluminium, glass, and paper have all been used with some success as supports for oil paint, but many of these require unusual preparation and conservation. The most used materials for the last 500 years have been wood and canvas. The first artists to develop the addition of drying oils to paint simply continued to make wood panels—in Italy using the readily available but somewhat unstable poplar, in Northern Europe using the heavier but more durable oak. As the Renaissance continued, so did the demand for ever more significant works that had to be transported further afield. Obviously, in these cases, wood was inconvenient and expensive.
Temporary decorative works had long been executed on linen; a cloth made from fibers derived from flax plants. It was gradually accepted that linen could support paint permanently. In a city like Venice, good quality linen was manufactured for sail canvas, and by the end of the 16th century, this material was a common support throughout Southern Europe. In the 19th century, cotton duck became available as a cheaper, stretchier, and generally inferior alternative.
In the 20th century, the increasing scarcity and expense of seasoned hardwood timber has been met by the appearance of various kinds of reconstituted wooden boards: Masonite, presdwood, Masonite, hardboard, and latterly, Medium Density Fiberboard. Most of these have proved suitable substitutes.
Generally speaking, a good support should have, to some extent, three characteristics:
- Absorbency—it should microscopically draw in a slight amount of the liquid binder of the paint.
- Tooth—it should have a slightly rough surface to allow the binder, when dry, to lock into it.
- Stability—in its dried state, it should be subject to no more movement than the paint it supports.
Clearly, metal, glass, and most stone do not have the ability to absorb but can be given tooth to allow for absorbency between the smooth surface and the paint. Wood and canvas naturally have both of these characteristics, to the point where their absorbency has to be diminished by subsequent preparation. But if poorly chosen, cradled, stretched, or kept in hostile surroundings, neither of these may be stable enough to avoid moving the paint layers to the point of wrinkling or cracking them. There is no perfect support material.
When choosing a support, think about the quality of the paint surface intended in the work. Art shops usually sell unprimed linen in a range of sizes of weave. Whilst exact handling of paint might seem to need the finest ‘portrait’ grade, many painters working on a large scale or with impasto would prefer the coarser ones. If the wobble of a stretched canvas is objectionable, then consider using wood, but remember that it is now difficult to obtain hardboard or MDF in some of the larger sizes. The traditional recourse for wood panels was to piece several smaller pieces together on a rear framework (‘cradling’) and then bridge the surface joints by glueing on canvas in strips or all over the surface (‘marouflaging’). This work can take some time.
2. Prepare the Support to Suit the Priming
Usually, artists think about the choice of priming as they choose support. The priming can be defined as the liquid application that, when dry, to some extent isolates paint layers from the support. This is necessary in the cases of wood and canvas, which, in their untreated states, are so absorbent that the oil binder would be sucked excessively into them, leaving a dull, matt paint surface and sometimes endangering the stability of the support. Metal, slate, and marble being virtually nonabsorbent do not require priming (which, if water-based, would not adhere well anyway). Still, these might require abrasion to give them a compensatory degree of tooth. Sometimes absorbent surfaces such as hardboard and natural hardwoods lack tooth, and light abrasion with fine sandpaper or silicon carbide paper is needed to remove their surfaces’ slickness.
The priming does not have to be a paint itself. For centuries, a glue made from rabbit skin, called size, has been applied as a preliminary layer to reduce, but not eliminate, the absorbency of wood and canvas. It is possible to start painting a picture straight onto this, though most artists would prefer to add a pigmented layer of some kind in order to increase the luminosity of their subsequent painting. The choice of primer layer crucially determines whether a preliminary layer of size is necessary. There are three kinds of primer in everyday use for oil painting:
This is essentially a layer or two of oil paint made more matt than is encountered in most tube colours by ensuring that the oil content remains proportionately low and/or by adding enough turpentine (N.B. artists’ quality, NOT decorators’) as a diluent. To satisfy the first condition, it is wise to use paint which has been prepared with priming purposes in mind; Michael Harding makes such a blend as his ‘Foundation/Priming White’. More on this is below. The critical point is that a preliminary layer or two of size is ALWAYS needed when applying an oil primer to an absorbent support such as wood or canvas.
This kind of size-bound white paint is often applied in three or more layers to rigid supports such as wood. There is evidence from analysis of 16th-century painting that it was applied to canvas as well. Still, modern authors debate whether it is wise to apply a preparation that dries so rigidly itself to a flexible support. At any rate, it would be prudent here to apply no more than one, or at most, two layers. But with all kinds of canvas and most kinds of wood, gesso requires that the same preliminary layer of sealant size be applied to the support to avert the excessive absorption of the size binding the gesso itself, which would leave an underbound surface to the latter, liable to dust off. The possible exceptions to this rule are those few kinds of wood which are only minimally absorbent, such as hardboard which has been “tempered” with oil in manufacture. However, this latter product seems virtually obsolete and may not offer a good surface for a water-based glue such as size.
The most recently devised of the standard primers, different kinds have been sold for over 30 years, and their durability on rigid supports at least seems very probable. Some might present more absorbent surfaces than oil grounds, but this can be adjusted by recoating with oil primer. It is important to note that acrylic primer should NOT rest on a size layer; such a coating would damage the primer’s adhesion to the support. Acrylic primers that give a toothier surface are often sold under the label ‘acrylic gesso’, but they bear no chemical resemblance to the true gesso described above.
It seems too obvious a point to make but remember that whilst an acrylic primer can be overpainted with oil paint, acrylic paints CANNOT be applied successfully to an oil primer since, being water-based, they would not adhere safely.
2.5 Preparing Sealant Size
As is evident, a semi-absorbent layer of size is necessary for both oil and gesso primings. Art shops will sell size of sufficient quality for this purpose; decorators’ “size” sold in DIY stores may not be good enough, and may not be rabbit-skin glue at all. Size is available in granule or in sheet form, often with instructions attached.
The dry size is generally added to water, in the ratio 8 parts water to 1 part size and allowed to soak overnight. The resulting gel-like solution should be gently warmed, and NOT boiled, until it has the consistency of a thick liquid. At this point it might be desirable to strain out a few little lumps that might remain unliquefied, by quickly decanting the size through a membrane of plain muslin cloth into another vessel.
The size should be sufficiently liquid to be evenly worked into the surface with a broad priming brush whilst still warm. On cooling, size will revert to a gel, and reheating cool size risks making it brittle on drying, so it is best to make a batch for one session only.
Some instructions will recommend stronger mixes when applying it to more absorbent supports such as wood or MDF. Some artists consider that two coats are preferable. It is important to avoid the extremes of an insufficient and excessively absorbent layer, or one which has no absorbency at all, and presents a glassy and impermeable surface.
3. Take Care with Priming
If a priming is faulty then your efforts to achieve desired artistic qualities may be constantly thwarted, and the survival of the work might be irreversibly impaired. Excessively slick or excessively suctive primings are uncomfortable to work on. A slick ground, lacking tooth and possibly absorbency as well, can induce later wrinkling and cracking in the paint film. A suctive ground, with excessive absorbency, will cause the paint to “sink” quickly to a dull and matted surface.
But the exact feel of a good ground is a matter of personal taste, and painters get quite neurotic in their response to its qualities. It is only possible to cater for one’s own preferences here by priming for oneself: ready-primed canvas or fibrous boards as sold in art shops are standardized products that may not be ideal.
The big DON’T here attaches to the use of decorators’ emulsions, undercoats or ordinary primers as used in house painting: these will normally only be durable to acceptable commercial standards, i.e. 5-10 years, and will often present surfaces which veer between the fiercely absorbent and the impermeable. Their use should be contemplated only after prior research, and for specialized purposes.
To take the three common primers again:
The advice here is fairly broad. It is clear that good drying and robust structure when dry is important for any primer. Lead Carbonate has been the most commonly used. But traditional advice recommended leaving Lead White primings to dry for at least 6 months in good light before painting on them. This somewhat diminished their popularity. Michael Harding, however, has produced a Foundation White which blends Lead and Titanium pigments with comparatively little oil content, which can be thinned with turpentine to ease application, and which in normal conditions can be dry enough to be painted on within a week.
The degree to which the neat primer should be thinned with turpentine before application is, again, a personal matter. The consistency of single cream has been cited as the norm, but some painters might find the resultant surface, when dry, rather too absorbent. But it is important, especially in the case of sized canvas, that the primer is thin enough to be worked well into the weave or grain of the support. And however thin, the mix with turpentine should be consistent, and accomplished on a surface before application, with a palette knife or spoon, in one batch for each coat. It should be applied with a priming brush or with a mottler in a systematic, area-by-area fashion, so as to avoid the unevenness resulting from evaporation of the turpentine.
Some painters will prefer two coats. In this case, the dilution with turpentine of the second coat should not be greater than that of the first (for an explanation of this, see below). A second coat can be coloured with additions of other paints, although, if adding these in quantity, it would be best to avoid very slow drying pigments.
There are various recipes for making this, but the general procedure can be suggested as:
- Prepare a small sample of size in the ratio 71/2- 8 parts water to one part size (i.e. about double the strength of sealant size) in the manner given in the section above. As with sealant size NEVER let it boil.
- Test the strength of this sample, when allowed to cool, by pressing the resultant jelly with a finger. If it is of satisfactory strength the gel should split with ragged and not with smooth sides, which would indicate that the size is too strong and that the above ratio should be slightly adjusted.
- Following the ratio which gives successful results, make up sufficient size for all the intended coats (i.e. at least 3).
- Add the dry pigment. Nowadays the pigment mix which gives the best results would be, typically, 405g-450g of chalk or whiting, with 45g-50g of zinc oxide powder, which is added to increase whiteness and consistency, and which also prevents mould infection if you wish to store any surplus gesso left. These dry powders should first mixed together well, then should be gently stirred into somewhat under 2 pints of a still warm size solution. The resulting gesso should be creamily liquid in consistency; it should feel right to the user. Both the above ingredients should be available at good art stores.
- Apply the coats of gesso in one session. There should be at least 3 coats, each applied as soon as, and not later than, the last is touch-dry (about 10-12 minutes). Take care not to allow the gesso to thicken up as subsequent layers go on; add a little warm water. Do not allow the gesso to dry on the brush; rinse it between coats. Do not trap bubbles in the applied coats; brush gently!
- The resulting surface should be satisfactory if it is thicker than 1.5mm, and should be dry within 48 hours. It may still need a light abrading with sandpaper or carbide paper to smooth it out.
- This surface, as it is, will also be too absorbent for oil paint, and so a weak sealant size solution (approx. 15 parts water to 1 part size) should be applied. Alternatively, to follow a17th century practice, a thin layer of linseed oil can brushed on to slake the surface, wiping off any excess later.
Part of the attraction of acrylic primer is its straightforwardness. You simply have to apply it to the support, possibly slightly thinning the first layer with water according to the instructions on the can. But you should check out the brand of primer you intend to use by painting up a sample and coating this with some oil paint; some brands, particularly when applied in 2 layers or less, have a reputation for being too absorbent to serve as good oil grounds. Since acrylic primers should not be applied to a sized surface, they can also seem to make an unsized canvas feel rather slack and rubbery, lacking the spring of its sized counterpart.
4. Fat over Lean
This old maxim concisely expresses how, generally speaking, a successful and durable painting is developed. I say “generally” because there are instances of great painters bending this maxim to conspicuous effect, but nevertheless to ignore it is to court disaster. It could be clarified by paraphrasing it as “Flexible over Inflexible”.
Paint as it is squeezed neat from the tube can be modified by adding to its oily binder. If a diluent like turpentine is added then the binder is thinned in what is called a “vehicle”. When wet such a paint is more fluid in handling, but when dry, most of the vehicle has evaporated, leaving the oily binder far more sparsely distributed through the pigment particles. As a result, the glossiness of surface caused by the dry oil is proportionately reduced, as is the physical strength and flexibility supplied by the gluing presence of the oil molecules, called polymers. A dry layer of such a diluted paint is, in consequence, matted in appearance and less flexible and more brittle in structure. It is said to be “lean”.
If, however, we add to the tube colour more oil, or a medium which contains oil and possibly resin ingredients, then we have added to the proportion of binder relative to that of the pigment. Assuming that whatever medium we use does not contain much of an evaporating vehicle, the resulting paint mix will dry with a greater amount than previously of the oil which gives it a surface gloss, and gives the dry film some flexibility. Although the pigment particles are thereby being more sparsely distributed than before, the surface quality imparted by the increased gloss usually optically intensifies their colorific strength. Such a glossier and more flexible paint is said to be “fat”.
As all painters realize, the rapid evaporation of the vehicle makes lean paint, with comparatively little oil, dry more rapidly than the same unmodified paint from the tube, and far more rapidly than fat paint, which has more oil still. The process of drying of oils is chemically complex (of which more later), but it entails quite a lot of initial rapid expansion of volume, followed by a slower, more gradual contraction (sometimes plotted as the “drying curve”). Paint moves as it dries, and the more oil it contains, the more and the longer it moves.
By now the sense of the maxim above must be clear: if you paint first with fat, flexible paint, and then paint over this with lean, inflexible paint, you are laying up the strong possibility of structural problems later. Of course, the visual result may indicate much more quickly that in some way you are abusing the material; the lean paint will probably dry as a mat, chalky and even resisting (i.e. “beading up”) film on top of more colourful and glossier underlayers. Later on, the lean paint, having quickly reached the end of its drying cycle, might be moved around so much by the still drying and moving bottom layers, that it will crack and shell off in pieces. Not a happy prospect.
So as a general rule, you should work over your layers of paint in the broad direction of: turpentine-thinned paint—neat tube paint—medium or oil enriched paint. In this way the hierarchy of drying speeds is respected, and the appearance of the work should not disappoint.
As a final observation, there are a few paints with very high oil content (i.e. more than average quantity of oil needed to make the pigment or dye component into a stable paint.) which, even as neat tube colour, dry very fat. If using these in underlayers, it would be wise to thin them first on the palette.
5. Use Appropriate Thinners
This is merely a short but necessary clarification to the previous references to diluents. “Thinners”, as a term, covers both what is used to clean brushes, and what is used to dilute paint for artistic purposes. Whilst standard decorators’ quality White Spirit is satisfactory for brush-washing at the end of the day (as long as it is rinsed out of the brushes with water immediately afterwards), it is not good enough to be used in the actual painting process. For this artists’ quality Turpentine is probably the best. Although White Spirit is produced to artists’ quality and sold as such, it lacks any trace of the gummy residue which all natural Turpentines possess, and which acts as an assisting binding agent when paint thinned with Turpentine is dry. By contrast, the oil of paint thinned with White Spirit is more molecularly disrupted, and dries to a slightly less stable film.
To risk repetition, decorators’ quality Turpentine, as sold in DIY stores, is NOT suitable as a diluent for artistic purposes.
6. Use Medium as a Help, not a Panacea
A medium can be defined as a pre-mixed blend of a drying oil, possibly a resin, and possibly a turpentine vehicle, which is commonly added to the later layers of paint in a work, so as to facilitate handling, and/or enable transparency of layers, and/or increase the gloss and colorific intensity of these layers. Most artists will already know this, and some reliable recipes for these are given under the section of this site Resources, in the pages Painting Mediums and Recipes.
The above advice constitutes more one of the Don’t’s of technique rather than the Do’s. Towards the end of the 18th century, as the gradual replacement of the workshop system by the art academies was almost complete, many painters were vaguely aware that a body of practical knowledge was being lost with this change. A corresponding notion developed that the great 16th and 17th century artists had achieved their effects by dint of some knowledge, now lost, of complex blends of oils, wax and certain hard resins, but if these could be reproduced in a medium, the “Venetian Secret”, as it was called, could be regained. To this end, through the next century, painters experimented ceaselessly with mixtures of heated oils and resins such as Mastic or Copal. Since no means of photochemical and spectroscopic analysis were really available for use on historical works until the late 1970’s, there was no evidence to contradict the claims of authenticity made for these preparations. You can still find some of them, often with picturesque names, on sale in some art shops today.
The results of modern analysis are at once more prosaic and more daunting. The binders in Renaissance and Baroque paintings are largely walnut and latterly linseed oil, with small amounts of pine resin or fir balsam being added on topmost layers of working. Copal and Mastic were not used, and, as the last 150 years have shown, are liable to yellow and darken conspicuously with time anyway. If the oils were treated at all, it was merely by being thickened by heating or sunlight alone (see the pages referred to above for directions for these procedures).
There is still, however a subtle hype surrounding some of the proprietary mixtures sold as paint media, offering artists a chance to import “old master” qualities into their work, at no extra artistic effort. As a result some painters have liberally added medium into every stage of their creations, painting fat over fat, with dire consequences in appearance and preservation.
So if you wish to select a commercially prepared medium, exercise the same degree of scepticism that you might with an ostentatiously labelled wine. Most manufacturers nowadays will to some extent list the constituents, and of these you can form some immediate judgements:
- Avoid those media containing Mastic and Copal if you wish to see your work not age into a warm “gallery tone”.
- Be circumspect with those which contain siccatives (agents to accelerate drying) and are of a dark appearance. The presence of driers can darken and embrittle dry paint films.
- Remember that, if present in large quantities, even Damar, (a reliable varnish resin discovered in the Dutch East Indies in the late 17th century) makes a dry paint film vulnerable to later applications of solvents, including turpentine and also those used by conservators.
- Be aware that Ketone, a varnish resin which yellows less than all others, including Damar, is also the most vulnerable to solvent action. This makes it a fine, easily replaceable varnish on its own, but for that reason it is not a robust constituent for a medium which should be a durable binder of paint.
- Remember that medium is there to provide possibilities of effects when the possibilities of normal tube colour have been exhausted. It would be unwise to envisage it as an addition to an underpainting or to preliminary paint layers. It has its place in the “fat over lean” order mentioned above.
- Buy a small quantity and try it out first. Does it feel right for you?
Since the choice of characteristics of medium is even more a matter of personal taste than that of priming, it might be better to make your own. As well as the recipes given in this site, one of the books cited in the Booklist pages, Formulas for Painters by Robert Massey, gives some interesting examples, although he tends to endorse the use of some of the resins about which we have reservations.
Finally, think of a medium as a facilitator of good handling; it will not guarantee it.
7. Slow Drying Paint
One can think of this as a particular application of the treatment of the flexibility/inflexibility rule given above. It is apparent to anyone using a variety of oil paints that, even in identical conditions, drying speeds will vary greatly between them. Burnt Umber can be touch dry by the end of a summer’s day, whilst Alizarin Crimson, unmixed, can take about a week.
Michael Harding, unlike some other paint makers, does not attempt to equalize the drying speeds of all the paints in the range by adding to them siccatives or retardants which will ultimately have an adverse effect on their appearance. Artists generally appreciate getting to know the idiosynchrasies of their colours, and do not wish to think of them as some single homogenous stuff which just happens to come in different colours.
Drying speeds can be correlated with different families of pigments: the earths are generally the fastest, followed by most of the inorganic metal compounds, with the organics generally, but not always, being the slowest. These speeds are also approximately related to the oil content of the paint, i.e. the amount of oil needed to mix the dry pigment into a useable paint. As a rule of thumb, those paints with high oil content will be the slower. But there are exceptions to this, notably Red 48 ( Scarlet Lake Deep) which is a high oil content organic, but dries very rapidly. As you may have noticed Michael prints a drying speed rating on all his labels to avoid misapprehensions, (very fast- fast- average- slow- very slow).
The commonsense to which this advice refers is applicable when using conspicuously slow drying paints, on their own and without much thinner, and then overpainting them with layers of much faster drying ones. If the underlayer is still only touch dry, and therefore still highly absorbent, then the result might be that it rapidly robs the overpainting of its binder, causing the latter to mat out or “sink”. In the longer term there is a risk that the underlayers will carry on moving long after the top layers will have hardened and become less flexible, causing the latter to crack.
Oil paint is a fairly robust material, and it does not do to overstate these risks, otherwise painters would be anxiously calibrating the drying rate of every touch. But at an extreme there is the historical example of the use of Bitumen, much favoured as an underpainting brown from the18th-19th centuries, but unfortunately a substance which never properly dried and which in fact became liquid again in high temperatures. Many works from this epoch have developed wide surface cracks, or even have slid away from their canvases in a buckling sheet. Thankfully, no paint as mobile can be found in the modern range of oil paints!
There are three specific points which may be useful advice:
- Remember that slow drying paints can be in effect accelerated by mixing them with faster ones, and that those paints which dry to a tough paint film will strengthen those which do not.
- When using black or white paints largely pure, remember that drying speeds to some extent suggest an order of application. It might not be advisable to make heavy use of Ivory Black over the slower drying Lamp Black, nor to paint solid sheets of Lead White over layers of pure Zinc or Titanium White.
- Those paints where the colour is supplied not by a pigment powder but by a liquid, called Lake colours, can sometimes be slow driers. In a modern range of paints, lakes are invariably organic, i.e. hydrocarbon-derived, and they have the strong tinting power typical of many organic dyes. (One the strongest, Phthalocyanine Blue Lake, exceptionally, dries fairly fast). But the slower drying ones may not be the wisest choice for an underpainting that will be worked upon rapidly.
Some years ago there were reports of a slightly mysterious tendency of the organic lakes to “strike through” or “bleed”, that is, to very gradually overpower other colours in a mix, or over a longer period of time, to leech into an overpainted layer and stain it. Since there do not seem to be present-day accounts of these phenomena, we suspect that they resulted from manufacturers’ inability to adequately stabilize the first generation of the organic lakes used in artists’ paints.